1. Field of the Invention
The present invention relates to alumina-based substrate materials for magnetic heads with low flying height and suitable for precision working such as ion beam etching (IBE). The materials can be used in thin-film head sliders for hard disk drives in which the head is situated at a very small distance from the recording medium or transverse pressure contours (TPC) for thin-film tape recording devices, for example.
2. Description of the Prior Art
A typical construction of magnetic heads has been a two-rail design called the taper-flat type. Such heads have been mechanically worked, for example, using the grinding wheel. Therefore, characteristics required of materials for such heads include (1) chipping resistance in cutting and (2) ease of machining in cutting and lapping, as well as (3) low friction in contact start-stop (CSS), which depends on the lubrication characteristics of the material.
The characteristic (1) above is indispensable to form an air bearing surface (ABS), i.e. the contact surface between the magnetic head and the recording medium. A head with numerous chippings on the ABS has shown poor flying height and lubrication characteristics. Also, a material that tends to chip while being machined has required a low machining speed for precision machining for the ABS, and therefore the throughput of heads using such a material has been low.
The characteristic (2) above also relates chiefly to the throughput: a material difficult to cut and machine requires a prolonged period for ABS machining.
The characteristic (3) above relates to the reliability of the head: a material with poor lubrication tends not to be very durable when subjected to repeated CSS.
On the other hand, thin film heads have progressed from the conventional standard slider (100% slider, with a 4-mm thick substrate) to the micro-slider (70% slider, with a 2.8-mm thick substrate), nano-slider (50% slider, with a 2-mm thick substrate) and pico-slider (30% slider, with a 1.2-mm thick substrate). The femto-slider (10% slider, with a 0.4-mm thick substrate) is under development for future use. Miniaturization of the head entails reduction of the flying height from 0.3 .mu.m through 0.2, 0.1 and 0.075 .mu.m to 0.05 .mu.m in the most recent models. The conventional two-rail taper-flat type sliders cannot cope with the low flying height necessitated by the miniaturization. To solve this problem, improved head sliders called TPC sliders have been introduced, which have a step 0.5-3 .mu.m in height at either end of the rail, or ABS, formed by a physical machining process such as IBE. Surface roughness is an important factor determining the throughput of this type of head, which should generally be 1/10 times of the machining depth, or 0.05-0.3 .mu.m. Substrate materials for this type of head should therefore give a sufficiently smooth surface after machining.
Another progress aiming at higher recording densities is the introduction of magnetoresistive (MR) heads. This type of head employs a magnetic film that requires magnetic annealing at high temperatures such as sendust, which anneals at 600.degree.-700.degree. C. This reduces the bond strength between alumina and TiC particles and, consequently, makes the TiC particles tend to pull out during lapping of the ABS. Therefore, a requirement for the substrate is that TiC particles do not come off even after magnetic annealing. Another problem brought about by magnetic annealing is that deformation of the substrate causes deterioration of positioning accuracy in photolithography and of magnetic characteristics of the head. The substrate is therefore required to show only a minimal deformation, if any, during magnetic annealing. In addition, characteristics of MR heads are strongly affected by surface roughness because of thickness as low as several tens of nanometers. The device is formed on a substrate sputtered with an alumina film. Too thick an alumina film may lead to large warp during magnetic annealing at high temperatures. On the other hand, if the alumina film is thin, its quality will be easily affected by defects in the substrate. Since the size of defects in the substrate depends on the crystallite size, the crystallites should be as fine and uniformly dispersed as possible.
The inventors have proposed in JP 86/50906 a sintered body consisting of 30-50 wt % of (TiC+TiO.sub.2) and alumina as the balance, to which is further added a small amount of a machinability agent such as MgO and Y.sub.2 O.sub.3, a sintering aid. The inventors have also disclosed in JP 90/62511 a magnetic head material consisting of alumina partially substituted with MgO or other machinability agents containing 5-45 weight % of titanium carbide, ZrO.sub.2, Y.sub.2 O.sub.3, and AlN. More recently, JP 94/2618 discloses a ceramic material for magnetic head sliders consisting of 100 weight parts of a mixture of alumina with 5-40 wt % of titanium carbide, 0.01-5 weight parts each of Mn and Ti oxides, and Ga, Ba, Ce and Ni oxides.
While these materials do satisfy the requirements described in (1)-(3) above, they are not satisfactory as the materials for TPC sliders or magnetoresistative heads. Their surface roughness after etching is as great as 0.2-2 .mu.m, or 1/5-1/4 times the etching depth (0.5-3 .mu.m), thus failing to give a stable flying height. When a substrate made of these materials is lapped to form the ABS after magnetic annealing, TiC particles pull out to deteriorate the surface roughness by a factor of 2-5, which causes problems such as poor CSS characteristics or head crash. A substrate 3 inches in diameter and 2 mm thick annealed at 600.degree.-700.degree. C. has 5-20 .mu.m of warp which greatly impairs positioning accuracy in photolithography and magnetic properties. These materials also have nonuniform grain size distributions which can bring about some large particles (20-100 .mu.m) in the substrate, which may introduce large defects. Therefore, the materials do not meet the requirements for advanced magnetic heads. This is because of poor distribution of TiC in the material due to the absence of oxygen and nitrogen in TiC.
In short, conventional Al.sub.2 O.sub.3 -TiC substrates do not satisfy the requirements (1)-(3) above and, additionally, do not satisfy such requirements as (4) low surface roughness after IBE, (5) small and uniformly dispersed crystallites, and (6) low internal stress. Therefore, it is difficult to produce sliders for high-density recording such as TPC sliders with high throughput using such materials.